Ballot initiatives that would have mandated GMO food labels failed in Colorado and Oregon.

It received a lot less attention than the Republicans' successful attempt to seize control of the U.S. Senate, but Tuesday's midterm elections may have marked the broadest attempt yet by critics of genetically modified food to advance their cause at the ballot box.

But opponents made local headway in Hawaii and California, where voters adopted two county-level bans on the production of genetically modified organisms—GMOs, or plants or animals genetically altered using DNA from bacteria, viruses, or other plants and animals.

The nominally bipartisan bill has had few co-sponsors, but a more business-friendly Washington could give it new life.

Genetically engineered foods must be labeled as such in 64 countries, but in the United States only Vermont has approved labels. Even there, the law doesn't take effect until July 2016—if it can withstand legal challenges. (Related: "Can Genetic Engineering Save the Florida Orange?")

Maine and Connecticut also have passed GMO labeling bills, but both remain dormant unless and until other states also pass similar legislation. Legislation to label genetically altered food has been introduced in 20 states.

But anti-GMO activists aren't about to let up.

"The question is, Will the groups continue trying in other states?" asked Erik Olson, senior strategic director of food and health at the Natural Resources Defense Council. "Clearly, they will."

Here's a closer look at what happened this week at the ballot box:

Colorado. Voters strongly rejected Proposition 105, which would have mandated labeling for genetically modified foods. The lopsided two-to-one vote came after chemical companies Monsanto and DuPont Pioneer contributed more than $16 million to defeat the measure.

"Those huge, out-of-state corporations fueled by greed may have won this election, but they have not truly won. Our voices can never be drowned out with truth on our side," said Larry Cooper, co-chair of Right to Know Colorado, which pushed for the measure.

Oregon. The vote was much closer in Oregon, but Measure 92 still failed, in what was the most expensive ballot initiative in state history. Nearly $27 million was spent by the same seed giants that toppled the Colorado initiative, dwarfing spending by supporters, which included the moguls behind Ben & Jerry's ice cream. The statewide vote followed an earlier one in Jackson County in May, when voters there approved a ban on GMO crops.

"The ballot initiatives in Oregon and Colorado prove that America has the best democracy that money can buy," said Dave Murphy, founder and executive director of Food Democracy Now.

Maui County, Hawaii. A ballot measure slapping a temporary ban on genetically engineered crops passed by a slim margin. The new law will prohibit the growth, testing, or cultivation of GMOs until environmental and health studies declare them safe.

Opponents, which included agribusinesses and family farmers, called the law flawed and said it would hurt the local economy. Indeed, the GMO seed corn industry on Molokai Island, which is part of Maui County, may be threatened as a result of the election.

But supporters, who were reportedly outspent by more than 87 to 1, hailed the result.

"Residents of Hawaii are acutely aware of their islands' ecological uniqueness, and they are willing to stand up to chemical companies to ensure that biodiversity is protected," said Ashley Lukens of the Hawaii chapter of the Center for Food Safety.

Despite the setbacks for GMO opponents, public distrust of genetically modified foods seems to be growing. And companies that make and sell food are paying attention.

General Mills early this year changed the recipe for Cheerios, so that they no longer include genetically modified ingredients. And national retailer Whole Foods said it would label genetically altered products sold in its North American stores by 2018.

"Tides are beginning to turn," said Nicole Darnall, a researcher at the Julie Ann Wrigley Global Institute of Sustainability at Arizona State University, in Tempe. "At the core of this issue is citizens' right to know and putting power into citizens' hands. History has shown that corporations can stall these sorts of measures, but ultimately they tend to get passed."